Downhole fluid analysis (DFA) is an important and efficient investigative technique typically used to ascertain the characteristics and nature of geological formations having hydrocarbon deposits. DFA is used in oilfield exploration and development for determining petrophysical, mine ralogical, and fluid properties of hydrocarbon reservoirs. DFA is a class of reservoir fluid an alysis including composition, fluid properties and phase behavior of the downhole fluids for characterizing hydrocarbon fluids and reservoirs.
Typically, a complex mixture of fluids, such as oil, gas, and water, is found downhole in reservoir formations. The downhole fluids, which are also referred to as formation fluids, have characteristics, including pressure, live fluid color, dead-crude density, gas-oil ratio (GOR), among other fluid properties, that serve as indicators for characterizing hydrocarbon reservoirs. In this, hydrocarbon reservoirs are analyzed and characterized based, in part, on fluid properties of the formation fluids in the reservoirs.
In order to evaluate and test underground formations surrounding a borehole, it is often desirable to obtain samples of formation fluids for purposes of characterizing the fluids. Tools have been developed which allow samples to be taken from a formation in a logging run or during drilling. The Reservoir Formation Tester (RFT) and Modular Formation Dynamics Tester (MDT) tools of Schlumberger are examples of sampling tools for extracting samples of formation fluids for surface analysis.
Recent developments in DFA include techniques for characterizing formation fluids downhole in a wellbore or borehole. In this, Schlumberger's MDT tool may include one or more fluid analysis modules, such as the Composition Fluid Analyzer (CFA) and Live Fluid Analyzer (LFA) of Schlumberger, to analyze downhole fluids sampled by the tool while the fluids are still downhole.
In DFA modules of the type mentioned above, formation fluids that are to be analyzed downhole flow past sensor modules, such as spectrometer modules, which analyze the flowing fluids by near-infrared (NIR) absorption spectroscopy, for example. Co-owned U.S. Pat. Nos. 6,476,384 and 6,768,105 are examples of patents relating to the foregoing techniques, the contents of which are incorporated herein by reference in their entirety. Formation fluids also may be captured in sample chambers associated with the DFA modules, having sensors, such as pressure/temperature gauges, embedded therein for measuring fluid properties of the captured formation fluids.
Downhole measurements, such as optical density of formation fluids utilizing a spectral analyzer, are prone to systematic errors in measurements. These errors may include variations in the measurements with temperature, drift in the electronics leading to biased readings, interference with other effects such as systematic pump-strokes, among other systematic errors in measurements. Such errors have pronounced affect on fluid characterizations obtained from the measured data. These systematic errors are hard to characterize a priori with tool calibration.